1. Field of the Invention
This invention relates to surface acoustic wave (SAW) devices and more particularly relates to SAW devices which are constructed to reduce or substantially eliminate the effects of certain undesirable strains which affect the operating characteristics thereof.
2. Description of the Prior Art
Devices incorporating SAW delay lines and resonators, are known and are useful as component parts of pressure sensors and high frequency oscillators, and the devices are useful in a number of other applications. Some of the outstanding features of SAW devices are their ability to provide real time delays of electromagnetic waves within relatively compact substrate materials, and their inherent rugged construction. These features permit the use of SAW devices in high vibration and G-force environments.
Typically, a SAW device may be constructed to provide a SAW delay line or resonator with fixed operating characteristics, which do not depend on a strain-inducing phenomenon, for use as part of a SAW oscillator or other such SAW system. Alternatively, SAW devices may be constructed for use in measuring parameters which induce strain in the substrate of the devices and correspondingly affect the surface wave properties thereof. For example, the propagation velocity of a surface acoustic wave and the length of the propagation path of a SAW delay line, are both functions of strain in the substrate at and near the surface, whereby the operating frequency, of an oscillator dependent on a SAW delay line located on the surface, varies with the strain at the surface. In general, any type of strain-inducing parameter, such as pressure, temperature, force, acceleration and other similar mechanical parameters can be measured by a SAW device fabricated with a suitably deformable substrate.
In SAW devices, since the surface acoustic waves exist near the surface of the SAW substrate with atomic particle motion confined to a depth of approximately one SAW wavelength (referred to herein as "at the surface"), the problem of surface contamination of the SAW substrate may be particularly acute. Therefore, there are many applications for SAW devices which require that the devices be vacuum encapsulated, such as in the manner shown in U.S. Pat. No. 4,213,104 to Cullen, et al. which relates to a vacuum encapsulation structure for a SAW device wherein the encapsulating structure is fabricated from the same kind of material as the SAW substrate and is attached to the substrate by a glass-frit seal.
The vacuum encapsulation of SAW devices presents stability problems, such as strain which is induced in the SAW substrate from thermal expansion and contraction of the encapsulating structure and which may induce undesirable strains in the SAW propagation region. Also, the seal between the SAW substrate and a vacuum encapsulating structure, electrical connections, and clamps and other such holders for retaining a SAW substrate in a vacuum encapsulation structure as part of a SAW device, may all act as sources of stress which induce undesirable strains in the SAW propagation region. These undesirable strains distort the SAW substrate resulting in undesirable changes in the SAW propagation path and velocity for the device.
Techniques have been developed for reducing or substantially eliminating undesirable strain effects due to expansion and contraction of the propagation region of a SAW device as a function of temperature. For example, U.S. Pat. No. 4,100,811 to Cullen, et al. relates to a SAW pressure transducer having two SAW oscillators whose outputs are subtractively combined to increase the pressure sensitivity of the SAW device while reducing the temperature sensitivity of the device. However, this particular SAW transducer structure is not designed to reduce or substantially eliminate the effects of undesirable strains which are due to bonds between a vacuum encapsulation structure, or such other similar structure, and the SAW substrate of the device.
U.S. Pat. No. 4,085,620 to Tanaka does relate to a strain relief technique for isolating a piezoelectric element from strains originating at a bond between a supporting member for the element and a metal substrate. The supporting member comprises a connecting section and a neck section of a relatively smaller diameter which is constructed to absorb stresses originating at the bond between the supporting member and the substrate. However, this strain relief technique is not satisfactory since it does not isolate the active region from stress in the substrate per se. Furthermore, the supporting member has a relatively complex structure which may increase the sensitivity of the piezoelectric element to vibrations.